Respiratory Mechanics and Lung Function

Respiratory Mechanics and Lung Function

Objectives

• Discuss the principles of static and dynamic lung compliance.

• Define airway resistance.

• Explain the relationship among ventilation, lung compliance, and airway resistance.

• Discuss Hooke’s law and its application to elastic recoil.

• Describe how pressure–volume curves illustrate airway dynamics.

• Explain the three patterns of gas flow through the airways and their impact on airway resistance.

• Explain the difference between positive pressure ventilation, negative pressure ventilation, and intermittent abdominal pressure ventilation.

• List examples of conditions that may cause shifts in pulmonary pressure–volume curves.

Mechanics of the Lung and Chest Wall

• Respiratory mechanics: Interaction of pressures and forces enabling lungs and chest wall to work together for breathing.

• Changes in lung physiology due to pulmonary disease are identified by changes in respiratory mechanics from baseline.

• Lung function: Expression of respiratory mechanics measured by pressure, volume, and flow.

• Lungs and chest wall act as two springs with counteracting pressures and forces.

• Inspiration: Muscles pull chest wall up and out, expanding lungs.

• Exhalation: Lungs pull inward, contracting chest wall.

• Factors influencing lung and chest wall interaction:

  • Flexibility/compliance for expansion.

  • Velocity and volume of airflow.

  • Pressure exerted by airflow.

  • Respiratory muscle activity.

  • Resistance to airflow.

  • Elastic recoil.

Compliance

• Compliance: Measurement of elastic capability of an organ or system; ease of stretching.

• Defined as change in volume per unit change in pressure.

• Both lungs and chest wall have elastic abilities, so their compliance can be measured.

• Total respiratory compliance: Compliance of lungs and chest wall combined.

Lung Compliance

• Also called lung distensibility.

• High compliance: Lungs easily inflated.

• Low compliance: Lungs difficult to inflate.

• High lung compliance:

  • Normal aging.

  • Emphysema.

• Low lung compliance:

  • Pulmonary fibrosis (stiffer lung tissue).

  • Increased pulmonary venous pressure (engorged vasculature).

  • Conditions restricting alveolar expansion (atelectasis, pulmonary edema).

• Calculation: Change in lung volume (L) / Change in transmural pressure gradient (cm H2O).

• Compliance = \frac{\Delta Volume}{\Delta Pressure}

• Example: Inhaling 500 mL air, intrapleural pressure changes from -5 to -10 cm H2O.

• Convert 500 mL to 0.5 L.

• Compliance = \frac{0.5 \, L}{-10 \, cmH2O - (-5 \, cmH2O)} = \frac{0.5 \, L}{-5 \, cmH2O} = -0.1 \, L/cmH2O

• Static compliance: Measured without airflow (end of inspiration or exhalation).

• Normal adult static lung compliance: 200 mL/cm H2O.

• Dynamic compliance: Measured during airflow (e.g., inhalation).

• Dynamic compliance is always lower than or equal to static compliance.

• Normal static to dynamic compliance ratio: 1:1.

• Adult lung compliance values are higher than in infants/children due to larger lung size and capacity for higher pressures/volumes.

• Specific compliance: Corrects for lung size differences.

• Specific \, compliance = \frac{Compliance}{FRC}, where FRC is functional residual capacity.

Chest Wall Compliance

• Elastic properties of chest wall bones and muscles affect ventilation.

• Measure of transmural pressure across chest wall compared to chest cavity volume.

• Can be static or dynamic.

Total Respiratory Compliance

• Lung compliance and chest wall compliance are each approximately 0.2 L/cm H2O when measured separately.

• They operate in opposition, partially canceling each other out.

• Normal total respiratory compliance (CT) is approximately 0.1 L/cm H2O.

• CT = CL + C_{cw}, where:

  • C_T = Total compliance

  • Er C_L = Lung compliance

  • C_{cw} = Chest wall compliance

Pressure–Volume Curves

• Changes in ventilation mechanics (volume, pressure, airflow) plotted on a graph.

• Volume on y-axis, pressure on x-axis.

• Illustrate mechanical properties of respiratory system.

Lung Compliance Curves

• Lung compliance is the slope of a pressure–volume curve.

• Curves show changes in lung compliance.

• Upward shift: Increased compliance.

• Downward shift: Decreased compliance.

Chest Wall Compliance Curves

• Changes in volume (y-axis) plotted against changes in pressure (x-axis).

• Chest wall movements counterbalance lungs.

• Decreasing transmural pressure reduces chest cavity size.

• Increasing transmural pressure expands chest wall and increases volume.

Elastance

• Lung compliance is the distensibility of lungs.

• Elastance/Elastic Recoil: Ability of lungs to spring back after expansion.

• EL = \frac{\Delta P \,(liters)}{\Delta V \,(cm \, H_2O)}

• Chest wall elastance can also be measured.

• Total elastance of respiratory system:

• ET = EL + E_{cw}, where:

  • E_T = Total elastance

  • E_L = Lung elastance

  • E_{cw} = Chest wall elastance

Hooke’s Law

• Spring stretches proportionally to applied force/load.

• Lung pressure is directly proportional to volume entering lungs.

• More pressure = more lung expansion = greater air volume.

• Volume increases with pressure until elastic limits are reached.

• Elastance is the inverse of compliance.

• High compliance = low elastance, and vice versa.

Clinical Focus: Lung Hysteresis

• Inspiratory and expiratory arches on pressure–volume curve show differences in lung volumes during inspiration vs. expiration.

• Lung volumes at a given pressure during inspiration are less than at the same pressure during expiration.

• Difference between the two curves is called hysteresis.

Pressure Gradients

• Pressure gradient: Change in pressure per unit distance.

• Air flows from high to low pressure.

• Measuring gradients is necessary for understanding normal breathing and managing mechanical ventilation.

• Baseline airway pressure of zero is reference point (1 atm or 760 mm Hg at sea level).

• Pressures below 1 atm are negative/subatmospheric.

• Pressures above 1 atm are positive/supra-atmospheric.

• Normal breathing:

  • Mouth (Pam) and nose (Pno) pressures are usually zero.

  • Body surface pressure (Pbs) is also zero.

  • Alveolar pressure (PA) changes with lung/chest wall movement.

• Inspiration:

  • Thoracic muscles lift chest, creating negative alveolar pressure.

  • Air moves from high pressure at mouth to low pressure in alveolus.

  • Airflow stops when pressures equalize.

• Exhalation:

  • Muscles relax, lung recoil compresses alveoli creating a positive pressure gradient.

  • Air moves out until pressure gradient is zero.

• Three pressure gradients:

  • Transrespiratory pressure gradient.

  • Transpulmonary pressure gradient.

  • Transthoracic pressure gradient.

Pressure Gradients: Transrespiratory Pressure

• P{rs} = PA − P_{bs}, where:

  • P_{rs} = Transrespiratory pressure

  • P_A = Alveolar pressure

  • P_{bs} = Body surface area

• Pressure required to inflate lungs and airways.

• Also called transairway pressure (PTA).

• Used in discussing positive pressure mechanical ventilation.

Pressure Gradients: Transpulmonary Pressure

• Pressure needed for maintaining alveolar inflation.

• Also called alveolar distending pressure.

• Increase in transpulmonary pressure increases alveolar volume.

• Excessive pressure can overextend alveolus.

• Insufficient pressure can lead to decreased alveolar volume and atelectasis.

• PL = PA − P_{pl}, where:

  • P_L = Transpulmonary pressure

  • P_A = Alveolar pressure

  • P_{pl} = Intrapleural pressure

Pressure Gradients: Transthoracic Pressure

• Total pressure required to expand/contract lungs and chest wall.

• Pw = PL − P_{bs}, where:

  • P_w = Transthoracic pressure

  • P_L = Transpulmonary pressure

  • P_{bs} = Body surface area

Airway Resistance

• Friction of airways and lung tissue to airflow during inhalation and exhalation.

• Factors affecting resistance:

  • Airway radius (inversely proportional).

  • Airflow velocity.

  • Airflow pattern.

  • Gas properties.

• Narrower airway increases gas velocity and turbulence, raising resistance.

• Low-density gases (e.g., helium) flow more easily, reducing turbulence.

• Heliox (helium and oxygen) is used to reduce resistance in conditions like asthma and COPD.

Airway Resistance: Patterns of Gas Flow

• Flow pattern influences RAW.

• Types: Laminar, turbulent, tracheobronchial (transitional).

• Laminar flow:

  • Uninterrupted, parallel movement of particles.

  • Smooth, even airflow.

  • Parabolic/cone-shaped movement.

  • Calm, relaxed, low-flow, low-pressure breathing.

  • Common in smaller airways (< 2 mm).

  • Associated with low RAW.

• Hagen–Poiseuille equation:

  • Quantifies pressure generated by laminar airflow.

  • \Delta P = \frac{8 \mu L \dot{V}}{\pi r^4}, where:

    • \Delta P = Pressure gradient

    • \mu = Viscosity of the air

    • L = Length of the airway

    • \dot{V} = flow rate

    • r = radius of the airway

  • Explains necessary pressure increases as air moves deeper into smaller airways.

• Turbulent flow:

  • Erratic, choppy air movement.

  • Molecules churn and bump into each other/airway walls.

  • Higher resistance due to collisions.

  • High-flow rate, high-pressure breathing.

  • Larger airways (> 2 mm).

• Tracheobronchial/Transitional flow:

  • Mix of laminar and turbulent flow.

  • Occurs at airway branches.

  • Laminar flow meets branching, creating resistance as molecules hit walls.

Airway Resistance: Reynolds Number

• Calculates airflow type.

• R_e = \frac{vd\rho}{\mu}, where:

  • R_e = Reynolds Number

  • v = velocity

  • d = diameter

  • \rho = density

  • \mu = viscosity

• Low Reynolds number = laminar flow.

• High Reynolds number = turbulent flow.

• R_e < 2000: Laminar flow.

• R_e > 4000: Turbulent flow.

• 2000 < R_e < 4000: Transitional flow.

Ventilation Time Constants

• Time (seconds) to inflate a lung portion.

• Time for alveolar pressure to reach 63% of change in airway pressure.

• Time constant = Airway resistance × Lung compliance.

• \tau = RC , where:

  • \tau = Time constant

  • R = Airway resistance

  • C = Lung Compliance

• Time intervals that determine rate of pressure and volume changes in lungs.

• Normal inspiration fills lungs predictably due to exponential nature of the filling process.

• Five intervals to inspiration:

  • Interval 1: 63% filled.

  • Interval 2: 86% filled.

  • Interval 3: 95% filled.

  • Interval 4: 98% filled.

  • Interval 5: 99% filled.

• Increased RAW and/or compliance = longer inflation time and time constants.

• Decreased RAW or compliance = rapid inflation and shorter time constants.

• Time constants measure effect of respiratory disorders on compliance.

• Restrictive disorders:

  • Decreased lung compliance and time constants.

  • Increased respiratory rate to maintain constant air volume.

  • Examples: ARDS, atelectasis, interstitial lung disease, pneumonia, pulmonary edema, pleural effusions.

• Obstructive disorders:

  • Increased RAW and time constants.

  • Slower, deeper breaths to move air past obstructions.

  • Examples: Asthma, chronic bronchitis, emphysema.

• Time constants are necessary for calculating dynamic lung compliance.

Representing Lung Dynamics with Graphics

• Airway dynamics illustrated with pressure–volume and pressure–time curves.

• Static and dynamic compliance plotted as pressure–volume curves (flow volume loops).

• Assess lung compliance and resistance during mechanical ventilation.

• Dynamic pressure–volume curve: bottom portion is inhalation, upper is exhalation.

• Static compliance curve bisects the two.

• Pressure–volume curve depicts airway pressures.

• PEEP (Positive End-Expiratory Pressure): Baseline pressure above zero.

  • Extrinsic PEEP: Intentionally added by ventilator operator.

  • Intrinsic PEEP (auto-PEEP): Incomplete exhalation, air remains in lungs.

• Pressure–time curve depicts pressure (y-axis) and time (x-axis).

• Peak airway pressure (Ppeak): Maximum pressure during inspiration.

• Plateau pressure (Pplat): Lower pressure after pause before exhalation.

• Plateau pressure correlates to elastic recoil and estimates transalveolar pressure.

• High transalveolar pressures increase risk of barotrauma and lung injury.

Clinical Focus: Positive Pressure Ventilation, Negative Pressure Ventilation, and Intermittent Abdominal Pressure Ventilation

• Mechanical ventilation: External device supports air movement into/out of lungs.

• Goal: Adequate gas exchange with minimal complications.

• Positive pressure ventilation:

  • Gas blown into airways to inflate lungs.

  • Achieved with manual resuscitator or mechanical ventilator.

  • Air delivered via mask, endotracheal tube, or tracheostomy tube.

• Negative pressure ventilation:

  • External application of subatmospheric pressure to chest wall.

  • Device pulls chest wall outward, causing air to rush in.

  • Examples: Iron lung, cuirass (chest shell).

• Intermittent abdominal pressure ventilation:

  • External pressure pushes up on diaphragm to support ventilation.

Summary

• Ventilation involves complex interactions of pressure, flow, volume, compliance, and resistance.

• Respiratory mechanics describes these interactions during a ventilation cycle.

• Compliance is the ability to stretch or expand (lungs and chest wall).

• Static compliance is measured without gas flow and illustrated as a slope on a pressure–volume curve.

• Dynamic compliance is measured during gas flow and shown as a curved inspiration line.

• Easily inflated lungs have high compliance; difficult lungs have low compliance.

• Lung compliance is proportionally equal in adults, infants and children.

• Specific compliance corrects for lung size differences.

• Elastance is the opposite of compliance; elastic recoil is the ability to spring back after expansion.

• Pressure gradients play a significant role in ventilation:

  • Transrespiratory pressure gradient (airway opening to alveolus).

  • Transpulmonary pressure gradient (alveolar space to pleural space).

  • Transthoracic pressure gradient (alveolar space to body surface).

• Airway resistance is friction of airways to airflow.

• Factors affecting airway resistance: airway diameter, gas velocity, airflow pattern, and gas properties.

• Flow patterns: Laminar, turbulent, tracheobronchial/transitional.

• Changes in respiratory mechanics (volume, pressure, airflow) can be plotted on pressure–volume or pressure–time curves.

• Compliance, peak inspiratory pressure, plateau pressure, and PEEP can be graphically represented.